1. Field of the Invention
This invention relates to a method for recovering petroleum from subterranean formations by the injection of flood water containing a surfactant or mixture of surfactants. More particularly, this invention involves the injection of a preflush fluid of adjusted salinity water having therein a thickening agent to efficiently displace the high salinity formation water.
2. Prior Art
Many subterranean petroleum-containing formations contain natural energy in the form of active bottom water drive, solution gas drive, or gas cap drive, in sufficient quantity to drive the petroleum to the production well from which it can be transported to the surface. This phase of oil recovery, known as primary recovery, recovers only a portion of the petroleum originally in place. When the natural energy source has been depleted, or in those formations where insufficient natural energy was originally present to permit primary recovery, some form of supplemental treatment is required to recover additional oil from the formation. Water flooding is by far the most economical and widely practiced supplemental recovery procedure. Water flooding is accomplished by injecting water into the formation via one or more injection wells. The injected water displaces and moves the petroleum toward one or more production wells, where it is transported to the surface. Water flooding is also quite inefficient, and approximately 50 percent or more of the original oil remains in the formation at the termination of conventional water flooding operations.
Numerous factors are responsible for the failure of water flooding to recover a high percentage of the oil remaining in the formation after primary recovery. A low viscosity fluid displaces a higher viscosity fluid quite inefficiently, because the low viscosity displacing fluid channels through the high viscosity fluid. The displacement efficiency can be related mathematically to the mobility ratio of the displacing and displaced fluids. Various additives have been proposed in the prior art to alleviate this problem. Hydrophilic polymers which increase the viscosity of the displacing fluid, improve the mobility ratio and decrease the tendency for the displacing fluid to channel or finger into and inefficiently displace the higher viscosity petroleum. U.S. Pat. No. 3,039,529 (1962) discloses the use of polyacrylamide polymer to increase the viscosity of injected water to improve the mobility ratio and hence the oil displacement efficiency of an oil recovery process. U.S. Pat. No. 3,282,337 describes the use of polyethylene oxide as a thickener for injection water for the same purpose.
The immiscibility of water and petroleum, and the high surface tension existing between water and petroleum also contribute significantly to the inefficient displacement of oil by water. The use of a surfactant to lower this surface tension will improve the displacement efficiency. For example, U.S. Pat. No. 2,233,381 (1941) discloses the use of polyglycol ether as a surfactant in an oil recovery process. U.S. Pat. No. 3,032,713 (1967) discloses the use of a particular petroleum sulfonate as a surfactant for oil recovery products. U.S. Pat. No. 3,468,377 describes the use of petroleum sulfonate having a specified molecular weight distribution as a surfactant for oil recovery.
The combined use of a surfactant solution to decrease the surface tension between the injected aqueous fluid and the petroleum contained in the formation, and a solution of a polymeric material to improve the mobility ratio and sweep efficiency provide a very efficient petroleum recovery process. For example, U.S. Pat. No. 3,477,511 (1969) describes the use of a surfactant solution followed by thickened water to displace the surfactant solution through the formation. Many other combinations of surfactants and water thickening polymers have been proposed, all sharing the common feature of specifying that the surfactant must precede the viscous fluid for optimum recovery efficiency.
Most of the chemicals which have been proposed in the above-cited references for improving the mobility ratio of the injected fluid and for reducing the surface tension between the injected fluid and the formation petroleum, require a specific, low salinity fluid environment to function effectively.
The salinity sensitivity of the most desirable surfactants for use in oil recovery has a substantial impact on the economics of a proposed supplement recovery operation employing a surfactant. While it has been generally recognized in the industry for many years that surfactants capable of reducing the interfacial tension between the injected fluid and the formation petroleum would improve the oil recovery efficiency of a supplemental oil recovery program, it has never been demonstrated that the additional oil which can be recovered under field conditions is sufficient to justify the cost of the surfactant. This is especially true because of the enormous quantity of surfactant which must be employed in a field, in order to have a significant effect on the displacement efficiency. If high formation water salinity results in a shift in surfactant choice to a higher cost material or if a greater concentration of surfactant must be used, the cost of a surfactant flood will be increased substantially. It is known, however, that many millions of barrels of oil remain unrecovered in a petroleum reservoir at the conclusion of conventional water flooding operations, and with an impending shortage of readily recoverable crude oil, it is becoming a matter of paramount national importance to devise a reasonably economical method of recovering this oil.